Numerical simulation of the microscale droplet evaporation process under transient radiative boundary conditions

High temperature droplet evaporation is a fundamental heat and mass transfer process that plays a critical role in a wide range of engineering and energy related applications, including fuel combustion, spray cooling, fire suppression, and radiative drying of porous materials. Under elevated temperature conditions, the evaporation behavior of droplets is governed by the strong coupling among convection, conduction, phase change, and radiation. In particular, radiative heat transfer becomes increasingly significant at high temperatures and can substantially alter the interfacial heat flux distribution, internal flow structures, and evaporation dynamics of droplets. As the droplet size approaches the characteristic wavelength of thermal radiation, wave optical effects emerge, leading to nonlinear absorption behavior and complex energy redistribution. These phenomena directly influence the evaporation rate, droplet lifetime, and thermal response of the surrounding medium. Therefore, an in depth understanding of evaporation mechanisms under transient radiative heating conditions, is essential for improving the predictive accuracy of thermal models and for optimizing the design and control of high temperature heat and mass transfer systems. To simulate the droplet evaporation process under transient radiative heating conditions, an independently developed in house integrated solver based on the LBM-FDM-FDTD framework is proposed, through which phase change, velocity field, temperature transport, and radiative distribution are solved simultaneously. Density distributions were used as inputs for temperature evaluation by the FDM and for electromagnetic field computation by the FDTD. The resulting temperature and radiative fields were subsequently provided to the LBM to update density evolution, and a fully coupled multiphysics computational model was thereby established. Time varying radiative boundary conditions can be imposed by this solver throughout the microscale droplet phase change process, and a theoretical foundation for the investigation of complex heat transfer mechanisms is thereby provided. In addition, radiative field distributions under different droplet sizes and incident wavelengths are simulated, and the results show that the incident wavelength significantly affects the internal energy distribution inside the droplet. The evaporation of a suspended droplet under different radiation source boundary configurations was simulated. The results indicate that, for an imposed radiative heat flux of 10 W/cm², the total evaporation time under all sided irradiation is reduced by 15.2% compared with the case without radiation. Under an identical total incident energy, the irradiation configuration alters heat flux distribution and the interfacial temperature distribution, and the evaporation rate is consequently regulated. In addition, the heat transfer mechanisms and their influences on evaporation were analyzed under different radiative intensities and vapor temperatures. These results are expected to provide a theoretical basis for the configuration design and performance optimization of radiative heating assisted evaporation processes, including high temperature drying of porous media and spray cooling systems.